In the automobile and trucking industries, it can sometimes be necessary or desirable to add a supplemental power source to a DC electrical system. In the trucking industry, for example, a solar charging system is sometimes added to the electrical system on the truck or on the trailer to counteract paralytic loads and/or to assist the charging of the battery. These solar systems typically include a solar panel and a solar charge controller connected to the battery. Various types of solar charge controllers are available that have been designed to provide multiple stage charging of the battery and, in some cases, maximize the power from the solar panel by controlling the voltage and current of the solar panel with the important objective of charging, but not overcharging, the battery. When these solar charge controllers operate in conjunction with the charging system on the vehicle (e.g., the alternator), the higher system voltage generated by the alternator can be interpreted by the solar charge controllers as a sign that the battery is fully charged, thereby triggering the solar charge controller to go to what is termed “stage 3” or “float mode.” In float mode, the voltage set-point of the solar charge controller is reduced to prevent over-charging of the battery. Because the voltage set-point is reduced and is lower than the voltage of the alternator, the solar system no longer contributes power to the system.
A similar situation is encountered when a solar charging system is added to a liftgate charging system that utilizes a DC-DC converter to charge an auxiliary battery. The alternator provides power to the DC-DC converter through long wires, and the DC-DC converter, in order to overcome the voltage drops in the long wires, steps up the voltage to more effectively charge the axillary battery. When the liftgate charging system is active, the solar charge controller can go to float mode and cease contributing power to the system, thereby leaving the alternator to generate the energy. The designs for solar controllers typically set the charging set-points for the voltage based on the stage of charging. For a three-stage solar charger, the first stage is a bulk charge stage where the solar charge controller directs the full charge capability of the solar panel to the battery until the battery voltage reaches a first set-point, typically around 14.7 Volts. In the second stage, or the top-off stage, the solar charge controller keeps the voltage at that point by reducing the current by utilizing pulse width modulation, or some other technique, until the current requirement reduces to a predetermined level, indicating that the battery is fully-charged. In stage three, float mode or maintenance mode, the charge controller reduces the voltage set-point to a value typically around 13.0 Volts with the objective of maintaining the charge on the battery.
In a typical electrical system of a vehicle, the electrical power is generated by the alternator. The alternator essentially converts mechanical energy to electrical energy. The engine rotates the alternator; the higher amount of power that the alternator generates, the more mechanical load the alternator puts on the engine. Studies show how much additional fuel is used by the engine to generate electrical power. See the white paper “Improving Alternator Efficiency Measurably Reduces Fuel Cost” by Mike Bradfield of Remy, Inc. This study shows how much fuel is used by the engine to produce mechanical energy, based on the efficiency of the engine, to produce power from chemical energy and the efficiency of the alternator to produce electrical energy from that mechanical energy. With an overall efficiency of 21% and a diesel fuel cost of $4/gal, the referenced white paper shows that the cost to generate electrical energy is about $0.51/kWh. This is many times higher than the typical residential utility rate.
The typical “key on” load of a class 8 truck, after the batteries are fully-charged, is about 280 Watts (20 Amps at 14 Volts). A 300 Watt solar panel could produce up to about 2 kWHr of electrical energy in a day. In the prior art, when the batteries are fully-charged, or if the solar charge controller goes into float mode, the solar energy is not captured.
Therefore, there is a need to reduce the cost of generating electrical energy in a vehicle.